1. Field of the Invention
The present invention concerns a device for generation of x-ray radiation, in particular for usage in a computed tomography apparatus, the device being of the type having an evacuable housing in which one or more cold electron sources are arranged as a cathode and at least one x-ray target is arranged as an anode, such that upon application of an electrical voltage between the cathode and the anode, electrons emitted by the electron source are accelerated in an electron beam onto the x-ray target.
2. Description of the Prior Art
Devices for generation of x-ray radiation are used, for example, in medical diagnostics in order to acquire radiographic images or, in the case of computed tomography (CT), images of the inside of the body of a patient. The requirements for x-ray tubes used in computed tomography have steadily grown with the manifold possibilities of computed tomography. Modern computed tomography systems thus require x-ray tubes that allow the x-ray current thereof to be modulated with high speed in order, for example, to be able to achieve an optimized dose modulation or operation at two different energies with an equilibrium photon flow (flux).
U.S. Pat. No. 5,105,456 discloses an x-ray tube for a computed tomography apparatus in which an electron source with thermionic emission is used. For the generation of x-ray radiation, the housing of this x-ray tube rotates with the x-ray target fastened therein, so that the electron beam emanating from the electron source (which is stationary) hits the x-ray target over time at different points. The rotating housing enables a better cooling of the x-ray target during the operation. U.S. Pat. No. 5,193,105 also uses an electron source operating by thermionic emission. In the x-ray tube of this patent, additional electrode systems (known as a RICE system (RICE: rotating field ion controlling electrode) and known as an ICE system (ICE: ion controlling electrode)) are arranged in the housing in order to reduce the proportion of positive ions in the region between the electron source and the x-ray target. The positive ions are captured in the electrode system This can ensue with a stationary alternating field or with an alternating electrical field. Positive ions are generated by impacts of the accelerated electrons with remaining gas molecules in the evacuated housing of the x-ray tube. These positive ions neutralize the repulsive forces between the electrons in the electron beam, such that a good focusing of the electron beam on the x-ray target is enabled in the focusing region. Since an optimally small focus can be achieved only with a sufficient divergence of the electron beam in the region in front of the focusing region, the positive ions in this region are unwanted since they would prevent the required expansion of the electron beam due to the repulsive forces of the electrons. Due to the aforementioned electrode arrangement, the proportion of the positive ions in this region can be reduced such that overall a sharper focus of the electron beam on the x-ray tube can be generated.
Due to the heating required for the emission of electrons, x-ray tubes based on thermionic emission exhibit a slow reaction time, a high energy consumption, and have a high space requirement. Such x-ray tubes are therefore less suited for the aforementioned modern CT applications.
In addition to thermionic emission sources, field emission electron sources (known as cold electron sources) are also known for the generation of x-ray radiation. For example, United States Patent Application Publication No. 2002/0094064 discloses an x-ray tube that can be used in a computed tomography apparatus. In this x-ray tube a substrate with a layer made from a field-emissive material (such as, for example, carbon nanotubes) is used as an electron source. The individual regions of this electron source can be selectively addressed by an applied electrode structure in order to be able to emit local electrons by means of the localized electrical field. The emission can ensue at a temperature of 300 K (cold emission) and be very rapidly activated and deactivated by the electrodes. X-ray tubes operating on the basis of a cold electron emission have the advantage of an exact control capability of the x-ray emission, such that the x-ray exposure can be reduced and the temporal resolution in the x-ray exposure can be increased. The field emission current in these x-ray tubes is controlled by the voltage applied to the electron source and not by the temperature, as in the thermionic emission. A pulsed x-ray emission with a variable pulse width and a high repetition rate therefore can be achieved by suitable control of the applied electrical field. The control voltage normally lies in a range between merely 50 and 100 V, such that a fast pulse sequence is simple to generate.
U.S. Pat. No. 6,760,407 also discloses such a device for generation of x-ray radiation for a computed tomography apparatus of the type described above. In this x-ray tube the x-ray source exhibits a curved surface that produces a focusing effect on the electron beam. An additional focusing device therefore can be foregone in this x-ray tube.
The lifespan of such cold electron sources in x-ray tubes, however, has conventionally represented a significant problem. The shortened lifespan is particularly caused by the ion bombardment of the sensitive surfaces of the cold electron sources as explained, for example, in Y. Cheng et al., “Electron field emission from carbon nanotubes”, C.R. Physique 4 (2003), pages 1021-1033 or in Y. Saito et al., “Cathode Ray Tube Lighting Elements with Carbon Nanotube Field Emitters”, Japanese Journal of Applied Physics, Vol. 37 (1998), pages 346-348. The ion bombardment is caused by the positive ions that arise due to impacts of the residual gas molecules remaining in the housing with the electrons of the electron beam. To increase the lifespan of the electron source, the maintenance of a very high vacuum of approximately 10−8 Torr [mmHg] in the housing of the x-ray source is therefore proposed. This can be achieved, for example, by the introduction of getter material in the evacuated housing. Such a high vacuum in high-power (high-capacity) x-ray tubes, as are required in CT systems, is very difficult to maintain due to the high anode temperatures. Furthermore, due to the space charge effects the high vacuum prevents the generation of a sharply-focused electron beam on the anode, since the neutralizing positive ions are absent.